Hitherto, there have been known various synthetic resin packaging containers for cooking by an electronic oven, which are sealed up after they contain retort food, and frozen food or the like, and heated in an electronic oven for cooking at the time of eating. However, when the packaging container is heated in an electronic oven, an inside pressure of the packaging container is increased by steam and the like generated from the food contained therein, the packaging container explodes, and the food is scattered. As a result, an inside of the electronic oven becomes a mess and harm such as a scald may be inflicted on a human body.
Therefore, before the above packaging container is heated in the electronic oven, it is partially unsealed or a hole is produced in the packaging container to discharge steam and the like, generated in the packaging container to an outside thereof so as to prevent the explosion of the packaging container.
However, this takes time and labor for an ordinary consumer. Since steam generated by heating in the electronic oven is discharged to the outside of the packaging container immediately, the packaging container has a disadvantage in that the effect of heating and steaming the contents of the container by steam lowers and the taste of the contents becomes worse.
To eliminate the disadvantage described above, there is proposed a packaging container having a back bonded part or a sealing fin which projects from a cover member to be bonded to a container body and is arranged across the cover member, and a release part or a weakly sealed part is formed in this projecting part (see Patent Documents 1 and 2, for example).
Patent Document 1: JP-A-2000-153885
Patent Document 2: JP-A-2003-200979
However, the packaging containers disclosed in these patent documents are containers in which a resin film constituting the cover member is lifted up to project from the top surface (side opposite to the container body) of the cover member to form the back bonded part or sealing fin, and this cover member is directly bonded to the body of the packaging container. Therefore, a space is formed between the base part (mating part) of the projecting part which is the back bonded part or sealing fin of the cover member and the container body, thereby causing imperfect sealing between the cover member and the container body at this position. As a result, there arises a problem in that a leak of the contents occurs during the distribution of the packaging container which contains the contents hermetically or during heating of the packaging container in the electronic oven for cooking.